For many years, law enforcement officers have used speed indicating radar guns to monitor and record the speed of vehicles travelling on roadways. Such speed indicating radar guns emit a directed radar signal towards a target and receive a reflected or re-transmitted portion of that signal from the target. Various types of radar detection devices exist for use in a vehicle to detect the presence of radar signals from such speed indicating radar guns. However, radar detectors typically are able to sense the presence of radar signals only from one direction--usually they are place or oriented within the vehicle to sense radar being transmitted from in front of the vehicle, and they rely on reflected radar signals within the vehicle or from a preceding vehicle to sense radar signals emanating from behind the vehicle.
Early model radar speed detector "guns" were somewhat unreliable and were prone to occasionally give erroneous indications of the speed of a vehicle. Recent advances in technology have caused these radar speed detection "guns" to be generally reliable. However, they are still not 100% reliable especially in case such as where two or more vehicles are in close proximity to one another.
Resultingly, over the last few years, vehicle speed indicating devices that employ laser light, hereinafter referred to as laser light speed indicator "guns", have been developed and are now being used by law enforcement officers. Such laser light speed indicator "guns" transmit pulses of laser light at a known frequency and with a known pulse repetition rate towards an object such as a vehicle. The laser light used is coherent infrared radiation. The laser light is reflected off the vehicle, typically off the bumper or fender of a vehicle. A portion of that reflected laser light signal--the echo--is returned to the laser light speed indicator "gun", which senses this returned signal or echo. The time from the initial transmission of a pulse until it's returned echo is received is indicative of the range of the vehicle from the laser gun; and the change of range with respect to time is the velocity of the vehicle. Thus, using known electronic means, the speed of the vehicle can be calculated from the pulsed laser light signal reflected and received by the laser speed measuring apparatus.